Full well capacity is the largest charge a pixel can hold before saturation which results in degradation of the signal. When the charge in a pixel exceeds the saturation level, the charge starts to fill adjacent pixels, a process known as blooming. The sensor also starts to deviate from a linear response and hence compromises the quantitative performance of the camera.
The dynamic range of a camera is typically defined as the full well capacity divided by the camera noise and relates to the ability of a camera to record simultaneously very low light signals alongside bright signals. Dynamic range, the ratio between the well capacity and noise floor, is a measure of how well a sensor can measure an accurate signal at low light intensities all the way up until it reaches full well capacity. Given a similar noise floor, the dynamic range of a sensing element is proportional to its well capacity, which is typically proportional to the size of such light sensing elements. Larger pixels have greater full well capacity and accordingly higher dynamic range.
In digital imaging, the dynamic range of a complementary metal-oxide-semiconductor (CMOS) sensor may, at times, be insufficient to accurately represent outdoor scenes. This may be especially true in the more compact sensors which may be used in mobile devices, for example in the camera on a mobile telephone. For example, a typical sensor used in a mobile device camera may have a dynamic range of approximately 60-70 dB. However, a typical natural outdoor scene can easily cover a contrast range of 100 dB between light areas and shadows. Because this dynamic range is greater than the dynamic range of a typical sensor used in a mobile device, detail may be lost in images captured by mobile devices.
Further, the trend in digital cameras is toward smaller pixels to drive megapixel count upward or to enable smaller sensor areas, and smaller pixels can suffer from more limited full well capacity than larger pixels. Particularly in mobile sensors, the trend in pixel size is to shrink the pixel size and also sensor area and attempt to maintain performance through advanced processing. Reduced full well capacity, reduced quantum efficiency, and reduced photosensitivity of the sub-micron pixel detector array has greatly degraded signal-to-noise ratio (SNR) and dynamic range of the image sensor. In addition, higher cross-talk of the reduced pixel size results in image quality issues for example poor modulation transfer function (MTF) and color fidelity.